Flexible motion segments of the human spine include at least a pair of vertebrae, each with a vertebral body, an intervertebral disc located between the vertebral bodies to provide a cushion and secure the vertebral bodies together, and a pair of facet joints, which are small stabilizing joints located posteriorly relative the vertebral bodies. The facet joints and the intervertebral disc, with the help of various ligamentation, function as a three joint entity to permit motion, but prevent excessive motion, over-twisting, or toppling over of the spine.
Each facet joint includes two articulating processes and a joint capsule. The surfaces of the facet joints are covered by an articular cartilage. The superior half of each facet joint is attached to the posterior portion of each vertebral body, on either side of the spinous process, and extends inferiorly. The inferior half of each facet joint extends superiorly from the inferior vertebral body to interact with the inferiorly extending, superior half of the facet joint.
Facet joints are in almost constant motion with the spine and may wear out or become degenerated due to aging, trauma, typical use and other factors. Facet joint degeneration or disruption is often preceded by instabilities or other degeneration of the anterior spine, often causing an overload on the facet joint. When facet joints become worn or torn, the cartilage may become thin or disappear and there may be a reaction of the bone of the joint underneath that produces overgrowth of bone spurs and an enlargement of the joints. The joint is then said to have arthritic changes, or osteoarthritis, that can produce considerable back pain during motion. This condition is also referred to as facet joint disease or facet joint syndrome.
Additionally, inflammatory reactions may occur when the cartilaginous surfaces of the facets become degraded or fissured, thereby leading to direct bone-on-bone contact and resulting in pain. Conventionally, patients have undergone interbody fusion surgery to alleviate such pain, in which the intervertebral disc space is removed and a spacer is inserted therein through which fusion of the adjacent vertebral bodies occurs, which by its nature is irreversible, non-motion-preservable, and oftentimes invasive. Commercially available and clinically tested solutions for facet joint replacement mostly feature dampening elements that are placed between pedicle screws. However, due to the limited bone stock a pedicle is capable of supplying, such interventions can result in follow-up surgery after further degeneration (the pedicles are needed for posterior stabilization in the case of subsequent fusion surgery). Furthermore, such interventions almost always include complete laminectomies, after which posterior fusion using trans-laminar techniques is extremely limited.
It is desirable to develop an implant for insertion into the facet joints between adjacent superior and inferior vertebral bodies to alleviate pressure on the painful articulating areas resulting from degenerating facet joints that may be implemented in a minimally invasive and bone sparing manner, and to provide the patient with pain-free mobility.